The invention disclosed and claimed herein generally pertains to a method for improving or enhancing thermal radiation transfer between selected X-ray tube components. More particularly, the invention pertains to a method for substantially increasing the ability of X-ray tube components to either emit or absorb thermal radiation in the Near Infrared Radiation (NIR) region, in order to enhance X-ray tube cooling. Even more particularly, the invention pertains to a method for forming a chromium oxide coating on components fabricated from high chromium content alloys specifically to increase the absorption of thermal radiation.
In a rotating anode X-ray tube a beam of electrons is directed through a vacuum and across very high voltage, such as 120 kilovolts, from a cathode to a focal spot position on a tungsten alloy anode target. X-rays are produced as electrons strike the tungsten target track, which is rotated at high speed, and are directed toward an X-ray transmissive window or port plate, provided in the tube housing. However, the conversion efficiency of X-ray tubes is quite low. More specifically, the total fraction of X-ray power emitted from the X-ray tube is typically less than 1% of the total power input. Thus, the remainder, in excess of 99% of the input electron beam power, is converted to thermal energy and contributes solely to heating the rotating anode assembly. Such energy must be dissipated in the forms of both thermal radiation and thermal conduction. Hot anodes in X-ray tubes emit thermal radiation with wavelengths of about 0.4 to about 25 microns, depending on temperature. This range is mainly contained in a region of the electromagnetic spectrum called the Near Infrared Radiation (NIR) region which covers wavelengths from about 0.7 to 25 microns. Failure to effectively remove or otherwise manage this fraction of non-productive energy limits tube performance, both by limiting continuous output power and by reducing the duration of transient, high power cycles. For rotating anode X-ray tubes, the added complexity of accelerated bearing wear is usually associated with a lack of effective cooling.
In a common arrangement, the X-ray producing components of a tube are contained within a tube housing, formed of stainless steel or other metal. Much of the excess heat is directed to the inner surface of the tube housing by means of thermal radiation. That is, a hot surface within the tube vacuum, such as the hot anode surface, will dissipate power to a cooler surface within the same vacuum space (e.g., the inner surface of the vacuum housing) by the emission of electromagnetic radiation. Since the radiation strikes the inner surface of the vacuum housing, it is very desirable to enhance the absorption of radiation at that location and minimize the amount of heat reflected back to the rotary anode and other internal tube components. The heat transferred to the housing may then be readily removed from the X-ray tube by means of a cooling fluid (usually, but not limited to, a dielectric mineral oil) which is circulated around the outer surface of the tube housing. Typically, the heat is carried by the cooling oil to a heat exchanger and dissipated thereby.
Generally, the efficiency of the thermal radiation transfer process can be engineered and exploited by adjusting the emissivity of X-ray tube component surfaces, such as the anode and housing inner surfaces, which are emitters and absorbers respectively, of thermal radiation. Herein, xe2x80x9cemissivityxe2x80x9d is defined as a measure of the efficiency of NIR absorption relative to the theoretically ideal xe2x80x9cblack bodyxe2x80x9d absorber. The emissivity will be expressed as a fraction of the theoretical ideal. For example, at a given wavelength, a surface with an emissivity of 0.5 will absorb 50% of the radiant power that a theoretically ideal black body is capable of absorbing. Accordingly, increasing the inner surface emissivity of the vacuum housing reduces the fraction of radiation power reflected thereby back toward the hot anode.
In metals, surface techniques that roughen the surface tend to improve the emissivity of the surface, especially in the critical NIR region of the electromagnetic spectrum of a hot rotating anode X-ray tube. In the past, methods such as grit-blasting, acid etching and plasma etching have been routinely used to increase surface emissivity. High emissivity coatings consisting of oxides, nitrides or carbides, have also been used and have been deposited by a number of methods, including plasma spray, chemical vapor deposition and physical vapor deposition. The type of process utilized and the materials selected are dictated by the application, the temperature range of interest and the environment to which the coating is exposed. However, prior art oxide coatings generally comprise nickel or iron oxides. It is very common for these oxides to reduce or evaporate when subjected to intense heat, that is, to give up oxygen and go back to base metal. Moreover, it has been found that coatings applied by plasma spray techniques tend to flake or crack off. It has also been found that efforts to increase emissivity by roughening a surface, such as by grit-blasting or acid etching, may leave an undesirable residue or may have non-uniform results over a surface.
The invention is directed to a comparatively simple technique for enhancing thermal radiation heat transfer between components within an X-ray vacuum tube, that is, from a hot component such as the rotating anode assembly to a cooler component such as the metal tube housing. These results are achieved by increasing the surface emissivity of the components, and more particularly by forming a chromium oxide coating thereon. By selective oxidation of the chromium alloying agent in a high chromium content alloy, in accordance with the inventive method described herein, it is possible to form refractive, oxide coatings that exhibit high absorption in the NIR region of the electromagnetic spectrum. This coating is tenaciously bonded to the base metal and does not evaporate or reduce at very high temperatures, such as 1000xc2x0 C., in vacuum. By oxidizing the surface of the vacuum housing, target cooling is enhanced significantly, as a greater fraction of the NIR power radiated thereto is absorbed rather than reflected back to the hot target. The vacuum housing temperature increases as it absorbs NIR, and is subsequently cooled by the lower temperature dielectric oil flowing over its external surface.
The invention is usefully embodied as a method for providing a selected X-ray tube component which has a desired thermal radiation transfer characteristic. The method comprises the steps of fabricating the component from an alloy containing a specified minimum amount of chromium, and then implementing a first heating operation which comprises heating the fabricated component in a dry hydrogen atmosphere, for a first specified time period, at a temperature selected from the range 1100xc2x0-1150xc2x0 C. Thereafter, a second heating operation is implemented, wherein the fabricated component is heated in a wet hydrogen atmosphere for a second specified time period at a temperature selected from the same range. Preferably, the method also includes the step of purging the fabricated component with a selected inert gas or nitrogen, between the first and second heating operations. This invention will solution anneal and transform alloys that respond to heat treating and age hardening (examples include martensitic stainless steels and superalloys). Subsequent thermal processing after coating may be necessary for alloys that fall under these categories. For example, precipitation aging of a superalloy could be accommodated in the same furnace during the cool-down step immediately after the wet hydrogen fire.
In a preferred embodiment, the component is fabricated from an alloy which is at least 12% chromium by weight. Higher chromium content alloys will yield higher emissivity values and form coatings that have greater thermal stability. Alloys that have chromium contents  greater than 18% (i.e. 300 series stainless steels) are considered the ideal embodiment of this invention. The dry hydrogen atmosphere of the first heating operation has a dew point which is less than xe2x88x925xc2x0 C., and the wet hydrogen atmosphere of the second heating operation has a dew point which is on the order of 18xc2x0 C. or greater. Preferably also, the fabricated component is selectively cooled between the first and second heating operations. Usefully, components selected for the method include all of the subassemblies that constitute the X-ray tube vacuum housing.
In another embodiment, the invention comprises a product formed by the method described above.